add Alt-[]/Cmd-[] for cycling through workspaces, shortcut cleanup

That wasn't quite what I meant to do, but I think the effect is the same.

* reduce precision of output floats displayed in GUI
 - puts is the only one I found, everything else seems to be rounding correctly already -- did I miss one?
* Properly report error when Ruby/Scsynth won't start up correctly (rather than hang)
 - I believe this is already in the Windows fixes: https://github.com/samaaron/sonic-pi/commit/0cd860beba17fe0e2671855bc651bf4ec814844d
* shortcuts for switching workspaces - Meta-] for right and Meta-[ for left numerically through workspaces
* hide phantom text+/- buttons by making them QShortcuts instead
* clean up shortcuts code